The invention relates to an evaporating fan, and in particular to an MVR fan (MVR=Mechanical Vapour Recompression) comprising a blade wheel and an electric motor to operate the blade wheel. The invention also relates to the blade wheel of this evaporating fan.
Evaporating fans are used in treatment plants based on the evaporation technique of various liquids (e.g. waste water) for providing the energy needed in the evaporation process. The energy is provided by raising the pressure of the vapour used as the substance in the process.
With an evaporating fan implemented according to the prior art, having a blade wheel of steel blades with separate roller bearings, it is at best possible to achieve a temperature rise of less than 8.degree. C. at one stage. The evaporating fan is used in processes in which vapour contains impurities and liquid drops, and therefore, the blade wheel has to be dimensioned to resist erosion and wear caused by liquid drops colliding at high speed.
The determining factor in the mechanical implementation of the evaporating fan of the prior art is the weight and strength of the steel blade wheel. Because of the heavy blade wheel compromises have to be made as regards the dimensioning of bearings. The bearings must endure the great stress put on them by the weight of the blade wheel, and also sustain the centrifugal forces resulting from the high rotation speed, and heat production in the roller elements. For the fan to operate smoothly, the critical rotation speed of the rotor system has to be above the operating rotation speed. In practice, this results in such dimensioning in which the roller bearings used are relatively large, and the heat generated in the bearings is led away by means of an effective oil circulation lubricating system.
Due to the above-mentioned features, the existing evaporating fans have to be provided with separate bearings, either roller or slide bearings, since the bearings of the electric motor do not endure the stress caused by a steel blade wheel. As a consequence, the evaporating fan arrangements are large and heavy, and cannot be economically integrated into the arrangements of evaporating plants.
An object of the present invention is to eliminate the drawbacks described above. This is achieved with an evaporating fan of the invention, characterized in that the blade wheel at least mainly consists of carbon-fibre-based composite material and is mounted directly on the shaft of the electric motor, and the tips of the blades of the blade wheel are provided with an erosion shield.
The blade wheel of the invention is comprised of a plurality of blades constructed of carbon-fibre-based composite material, and the tips of the blades are each provided with an erosion shield
By making the blade wheel of an evaporating fan or its essential components of carbon-fibre-based composite material the weight of the blade wheel can be reduced to a fraction of the weight of a steel blade wheel. A blade wheel made of this material can be dimensioned to be as strong as the steel blade wheel or even stronger. The light composite blade wheel can be mounted directly on the shaft of a standard electric motor without the stress on the bearings increasing too high. By means of an A/C inverter a fan implemented in this way can be rotated up to the maximum rotation speed given by the manufacturer of the bearings, and thus the pressure and temperature can be raised as high as with the conventional evaporating fan arrangement, or even higher.
Compared with the conventional evaporating fan, the evaporating fan of the present invention allows to dispose of 2 bearings with their casings and circulation lubricating units as well as of couplings between the electric motor and the blade wheel construction since no separate bearings are needed. An arrangement of this kind is compact and very light. It is easy to integrate such a fan into the heat exchanger of an evaporating plant, and thus fewer channels and less space are needed. The composite blade wheel will resist the erosion and wear caused by liquid drops if its critical sections are covered, preferably e.g. with a bent steel plate shield, or coated with appropriate coating, preferably e.g. with ceramic coating.
In the following, the invention will be described in greater detail with reference to the accompanying drawings.